(1) Field of the Invention
The invention relates generally to amplifying circuits and more specifically to an amplifying circuit having a multi-stage, negative feedback loop.
(2) Description of the Prior Art
Nearly every instrument or electronic control system includes an amplifying circuit for strengthening either the transmitted or the received signal of the system. Sonar devices, for example, require amplification of echoes detected by a scanning receiver. Ideally, the amplifying circuit should achieve a high gain in the signal without distortion over a broad range of frequencies. Often, amplifying circuits may require more than one stage of amplification to achieve the desired gain and necessary frequency response.
Multi-stage amplifying circuits may have either an open-loop or a closed-loop architecture. One shortcoming of high-gain open-loop, multistage amplifiers is sensitivity to capacitive coupling and other disturbance signals. Capacitive coupling may be caused by stray capacitance in printed circuit traces or other types of lead wiring. The stray capacitance can produce degraded pulse response and "ringing" from undesired resonances. As a result, open-loop multi-stage amplifiers with large amounts of gain often suffer instability. Other shortcomings of open-loop amplifiers include amplification of the direct-current offset. This amplification reduces signal headroom at the output.
Multistage closed-loop circuits address the shortcomings of open-loop architecture. Such circuits may have local feedback in each stage of amplification. Multistage feedback, however, requires absolute loop stability. Because a stable loop requires compensation of the accumulated phase responses of the individual stages of amplification, multistage feedback is difficult to implement over more than two stages without complex circuitry.
Related art includes U.S. Pat. No. 4,074,204 by Broburg et al. Broburg et al. disclose a multistage amplifier having three operational amplifiers and two segregated passive networks. The passive networks shape different portions of the amplified signal and are not optimized for high gain, low noise or loop stability. Also, Broburg et al. do not mention noise Figure optimization as it relates to gain distribution.
Other related art includes U.S. Pat. No. 3,863,173 by Scheib et al. Scheib et al. disclose a multistage, negative feedback amplifying circuit. The circuit has a group of cascaded amplifiers coupled to one another through filter sections and also has an external direct-current negative feedback loop comprising a feedback resistor connected between the output of the last cascaded amplifier and the inverting input of the first cascaded amplifier. The circuit minimizes the inherent voltage offset of each amplifier, but does not provide any gain.